This project is a continuing search for a useful physiological function of calcitonin in mammals including man. In the past year we have proposed such a function; namely, the conservation of dietary calcium by the coordinated action of calcitonin and parathyroid hormone secreted during the digestive sequence following feeding. Current emphasis is concerned with the effects of calcitonin infusion with and without concurrent calcium infusions, and with the study of plasma and urinary calcium and phosphate changes during the feeding period in rats. The parameters studied are changes in plasma 45Ca and 32P concentrations, urinary calcium loss following eating, and the tensile strength of bone. Comparison is made between normal rats, thyroparathyroidectomized animals and rats with parathyroid gland autotransplants and with or without thyroids. Electron microscopic studies will concentrate on changes produced by both hormones in the osteocyte-lining cell complex of bone. BIBLIOGRAPHIC REFERENCES: Samuel H. Doppelt and Roy V. Talmage. Calcitonin and the bone fluid compartment. Clinical Orthopaedics, in press, 1976. Roy V. Talmage, Samuel H. Doppelt and Jan H. Postma, Jr. Observations on the relationship of parathyroid hormone and calcitonin to plasma and liver phosphate. Proc. Soc. Exp. Biol. Med., in press, 1976.